1. Molecular organization of Moloney sarcoma virus was studied by nucleotide sequence analysis. The entire M-MSV including the terminal redundancies of this genome were sequenced. These studies revealed several important biochemical features of this viral genome which enables us to understand the transcription, translation and mechanisms of integration of this viral genome. 2. Abelson-MuLV genome in its integrated form was cloned in E. coli. Biochemical analysis was carried out to understand the structural organization of this viral genome. 3. Studies were carried out to understand cellular targets for in vivo transformation by two clonal replication competent type C viruses--Moloney and Rauscher MuLV. Moloney MuLV was found to induce T-cell tumors while Rauscher MuLV produced tumors of B-cell origin. The two leukemia viral genes have been molecularly cloned and recombinants between the two viruses using the cloned DNA molecules are currently being generated to understand the tissue specificity of these two viruses.